The characteristics of semiconductor materials such as silicon, germanium and gallium arsenide and other semiconductors have been exploited to form a large variety of useful devices in the fields of electronics, communications, electro-optics, and nano-technology. Ultra shallow junctions are required for future semiconductor devices. The formation of a shallowly doped semiconductor having an abrupt interface is difficult. Prior art methods have employed ion implantation techniques using very low energy conventional ions. Typical ion implanters suffer from greatly reduced ion beam currents at very low energies and therefore result in a low processing throughput. In efforts to increase the throughput of shallow doping processes, alternative techniques have been developed. These include plasma ion doping and decaborane ion implantation (or similar molecular implants). All these methods require a pre-amorphizing implant to prevent ion channeling of the doping implant species, which would otherwise produce undesirably deep junctions. A pre-amorphizing implant is an ion implantation step done prior to a doping step for the purpose of damaging the region to be doped so as to reduce or eliminate the crystallinity of the region to reduce the degree of channeling of the dopant, which would otherwise result in a dopant depth distribution with an undesirably deep tail due to channeled dopant atoms. Such pre-amorphizing damage implants are often done with inert gases like Ar or Xe or with non-electrically active ion species like Si or Ge. For some semiconductor devices, it is desirable to dope the semiconductor material with, for example, boron at very high doping concentrations. With conventional ion beams, including even molecular ion beams (decaborane, for example) the development of high doping levels using the low beam currents available at the very low ion energies required for shallow junction doping is a low productivity process. Additionally, the solid solubility limit of the dopant in silicon has been an upper limit for effective doping. Prior art indicates that the solid solubility limit of boron in silicon can be increased by introducing germanium atoms to the silicon.
The use of a gas-cluster ion beam (GCIB) for etching, cleaning, and smoothing surfaces is known (see for example, U.S. Pat. No. 5,814,194, Deguchi, et al.) in the art. GCIBs have also been employed for assisting the deposition of films from vaporized carbonaceous materials (see for example, U.S. Pat. No. 6,416,820, Yamada, et al.) For purposes of this discussion, gas-clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such clusters may consist of aggregates of from a few to several thousand (or even tens of thousands) molecules or more, loosely bound to form the cluster. The clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams of controllable energy. Such ions each typically carry positive charges of q·e (where e is the magnitude of the electronic charge and q is an integer of from one to several representing the charge state of the cluster ion). The larger sized clusters are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster energy. Consequently, the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes ion clusters effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage and/or dopant channeling that is characteristic of conventional ion beam processing.
Means for creation of and acceleration of such GCIBs are described in the reference (U.S. Pat. No. 5,814,194) previously cited and which is incorporated herein by reference. Presently available ion cluster sources produce clusters ions having a wide distribution of sizes, N, up to N of several thousand, or even tens of thousands, the distribution typically having a mean cluster size N at greater than 200, and commonly greater than several thousand (where N=the number of molecules in each cluster—in the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as either an atom or a molecule and an ionized atom of such a monatomic gas will be referred to as either an ionized atom, or a molecular ion, or simply a monomer ion—throughout this discussion).